The TSA, which oversees U.S. airport security, comes in for a lot of criticism — much of it deserved. But sometimes commentators let their dislike for the TSA get the better of them, and they offer critiques that don’t stand up logically.

A good example is yesterday’s USA Today article on TSA’s behavioral screening program, and the commentary that followed it. The TSA program trained screeners to look for nervous and suspicious behavior, and to subject travellers exhibiting such behavior to more stringent security measures such as pat-down searches or short interviews.

Commentators condemned the TSA program because fewer than 1% of the selected travellers were ultimately arrested. Is this a sensible objection? I think not, for reasons I’ll explain below.

Before I explain why, let’s take a minute to set aside our general opinions about the TSA. Forget the mandatory shoe removal and toiletry-container nitpicking. Forget that time the screener was rude to you. Forget the slippery answers to inconvenient Constitutional questions. Forget the hours you have spent waiting in line. Put on your blinders please, just for now. We’ll take them off later.

Now suppose that TSA head Kip Hawley came to you and asked you to submit voluntarily to a pat-down search the next time you travel. And suppose you knew, with complete certainty, that if you agreed to the search, this would magically give the TSA a 0.1% chance of stopping a deadly crime. You’d agree to the search, wouldn’t you? Any reasonable person would accept the search to save (by assumption) at least 0.001 lives. This hypothetical TSA program is reasonable, even though it only has a 0.1% arrest rate. (I’m assuming here that an attack would cost only one life. Attacks that killed more people would justify searches with an even smaller arrest rate.)

So the commentators’ critique is weak — but of course this doesn’t mean the TSA program should be seen as a success. The article says that the arrests the system generates are mostly for drug charges or carrying a false ID. Should a false-ID arrest be considered a success for the system? Certainly we don’t want to condone the use of false ID, but I’d bet most of these people are just trying to save money by flying on a ticket in another person’s name — which hardly makes them Public Enemy Number One. Is it really worth doing hundreds of searches to catch one such person? Are those searches really the best use of TSA screeners’ time? Probably not.

On the whole, I’m not sure I can say whether the behavioral screening program is a good idea. It apparently hasn’t caught any big fish yet, but it might have positive effects by deterring some serious crimes. We haven’t seen the data to support it, and we’ve learned to be skeptical of TSA claims that some security measure is necessary.

Now it’s time for the professor to call on one of the diehard civil libertarians in the class, who by this point are bouncing in their seats with both hands waving in the air. They’re dying to point out that our system, for good reason, doesn’t automatically accept claims by the authorities that searches or seizures are justified, and that our institutions are properly skeptical about expanding the scope of searches. They’re unhappy that the debate about this TSA program is happening after it was in place, rather than before it started. These are all good points.

The TSA’s behavioral screening is a rich topic for debate — but not because of its arrest rate.